Several types of systems may be used for navigation including global positioning systems (GPS) and inertial navigation systems (INS). However, both GPS and INS may suffer from inaccuracies or unreliability when used under certain conditions. For example, some less sophisticated GPS receivers may be less accurate than desired due, for example, to signal noise, or may be unavailable when signals from GPS satellites are blocked by structures, such as buildings and tunnels, or when signals are intentionally blocked. INS may suffer from inaccuracies due to drift, which are accumulated inaccuracies in the position determination that increase over time. As a result, INS requires frequent supplementation from another form of navigation system to reset the position determination. As a result, GPS and INS may not be sufficiently accurate or reliable for some implementations.